[Bug target/115756] default tuning for x86_64 produces shifts for `*240`

[liuhongt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115756

--- Comment #3 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
Current rtx_cost for imulq in generic_cost is COST_N_INSNS (4), make it as
COST_N_INSNS (3) could generate imulq.


  {COSTS_N_INSNS (3),                   /* cost of starting multiply for QI */
   COSTS_N_INSNS (4),                   /*                               HI */
   COSTS_N_INSNS (3),                   /*                               SI */
   COSTS_N_INSNS (4),                   /*                               DI */